Human-enhanced tsunami hazard in large lakes.

Worldwide, human-induced erosion enhances sediment delivery to lake deltas, increasing their susceptibility to collapse. In Lake Iseo, northern Italy, two millennia of intensified land use tripled sediment flux, oversteepening the delta and preconditioning it for failure. Here we show from geological records and numerical simulations that earthquakes in 1117 CE (Mw 6.5) and 1222 CE (Mw 6.0) triggered major subaqueous delta collapses, generating massive tsunamis with maximum run-ups of ~50 m and ~13 m, respectively. Historical accounts from the Vatican Library and archeological evidence support these reconstructions. Since the last collapse, eight centuries of human-induced erosion have rebuilt the delta, leaving shoreline populations highly vulnerable to future earthquake-triggered failures. A global lake synthesis showed that human-driven erosion has reached its highest levels since the Industrial Era, progressively overloading deltas worldwide; thus, similar catastrophic failures may become more likely in the future. These findings reveal a persistent, human-amplified mechanism of lacustrine tsunami generation and intensity with major implications for hazard assessment.